1SS3.] 293 [Hyatt. 



whorls are very involute, and there is a deep, impressed zone of 

 involution on the dorsum. The young sutures are probably 

 similar in outline to those of Discitoceras, but in the adults there 

 are ventral saddles, according to DeKoninck. Type, Phac. 

 £N~aut.) oxystomum, sp. DeKon. Calc. Carb. pi. 17. 



Aphelaeceras, 1 nobis, includes Carboniferous species allied to 

 Discitoceras until a late stage, but the whorls are more compressed 

 laterally, have hollow abdomens in later stages and adults, and 

 sides more convergent. The forms are gyroceran, and have no 

 impressed zone on the dorsum, which is gibbous, and sometimes 

 projecting along the centre. Sutures, and living chambers and 

 apertures similar to those of Discitoceras. The young appear to 

 have median dorsal saddles, which become divided by slight dorsal 

 lobes during growth. In the adults there is a dorsal lobe, but the 

 median saddle appears to have been absent in the adolescent stage 

 of the single species we have examined. This genus also includes 

 nautilian species, which differ from the typical Aphelaeceras in be- 

 ing involute and in having an impressed zone on the dorsum, but 

 the dorsal lobe similar. Aph. (Naut.) difficile sp. DeKon. Calc. 

 Carb. and disciforme, sp. M. et W., Geol. 111., Vol. 5, pi. 18, are 

 members of this subdivision. 



Subclymenia, D'Orb. Prod, de Pal., Vol. 1, p. 114, differs 

 from Discitoceras in the sutures, and position of the siphon. 

 The sutures have a deep V-shaped ventral, and acute, linguiform 

 first pair of saddles, first pair of lateral lobes narrow, a second 

 pair of small, Literal saddles near the umbilical shoulders, and 

 dorsal saddles, divided by shallow annular lobes with a minute 

 median saddle. The abdomens are hollow and the dorsal region 

 gibbous, as in the adults of Aphelaeceras. The siphon is near the 

 venter, but the funnels do not approach near enough to inter- 

 rupt the sutures, or affect the depth of the ventral lobes. But 

 one Carboniferous species is known, Subcly. evoluta, sp. Phil., De 

 Kon. Calc. Carb., pi. 45. 



Triboloceratidae. 

 This family includes shells, which at some stage have longitu- 

 dinal ridges rendered subspinous by the transverse striae. The 

 whorls in section tend to become depressed, and in the higher 



1 ' A4>sXts, smooth. 



